This invention relates to light beam scanners in which a light beam is one-dimensionally deflected to form a scanning line on a surface to be scanned and the surface is moved substantially perpendicular to the direction of the scanning line. This X-Y scanning forms a plurality of parallel scanning lines on the surface. More particularly, this invention relates to a light beam scanner in which the parallel scanning lines are formed at predetermined equal intervals.
A device for two-dimensionally scanning a surface to be scanned, with a light beam is known in the art. In this system the light beam is deflected in one direction by an optical deflector (hereinafter referred to as "a main scan" when applicable), and the surface to be scanned is moved substantially perpendicular to the direction of the scanning line formed on the surface (hereinafter referred to as "an auxiliary scan" when applicable). In the auxiliary scanning mechanism of the conventional light beam scanner, a continuously rotating motor is employed as the drive source and the rotation of the motor is converted into the linear motion of the surface to be scanned. In order to convert the rotational motion into linear motion, a lead screw is used, or the surface to be scanned is pulled (by using a belt or a wire).
If, in such a light beam scanner, the main scan is carried out with a constant period, but the speed of the auxiliary scan, i.e., the movement speed of the surface to be scanned is not strictly constant (hereinafter this being referred to as "irregular feeding" when applicable), then the scanning lines are arranged at irregular intervals. That is, the scanning lines are arranged coarsely and finely. If, in such a case, information inputted from outside is recorded as an image with a light beam, then the recorded image is such that coarse and fine intervals of the scanning lines can be observed as changes in density. Hence, the irregular intervals of the scanning lines appear as a stripe pattern. Therefore, the quality of the image is considerably lowered. This is especially true when the interval of the scanning lines is so short that the scanning lines are overlapped. Then the region in the image where the scanning lines are overlapped is higher in density than the region in the image where the scanning lines are coarsely arranged. As a result, the stripe pattern is emphasized. This is serious especially in an image which is recorded with continuous tone.
This problem is described in detail in "Various Problems in Cylindrical Facsimile Auxiliary Scan" of the National Technical Report, Vol. 22, No. 5 (Oct. 1976) pp. 550 to 558, or in "Visibility and Correction of Periodic Interference Structures in Line-by-Line Recorded Images" of the Journal of Applied Photographic Engineering, Vol. 2, No. 2 (Apr. 1976) pp. 86 to 92. It has also been confirmed by experiments done by the inventors.
In order to eliminate the deterioration of the image recorded by the scanning lines, which is caused by the occurrence of the stripe pattern, a variety of methods have been proposed. In one of the conventional methods, the auxiliary scan mechanism is operated with high accuracy to minimize the irregular feeding. In another conventional method, an optical deflector is additionally used to deflect the light beam in the auxiliary scan direction. The position of the light beam is corrected so that the scanning lines are arranged at equal intervals.
Concerning the former method, a lead screw system is known as best in accuracy. However, this system must satisfy the requirements that the fluctuation in rotation of a motor which is a drive source must be minimized. Each essential component such as a lead screw or a lead nut must be manufactured with high accuracy and the combinations of the essential components such as the junction of the motor and the lead screw and the engagement of the lead screw and the lead nut must be achieved with high accuracy. Thus, the light beam scanner according to the former method is necessarily expensive. Because the accuracy in combination of the lead nut and screw is increased, the load torque exerted on the motor is increased. Furthermore, if the installation accuracy is not sufficiently high, then the load torque is significantly changed. That is, the functional burden to the motor is increased.
The latter method is effective for the case where an optical deflector (such as a rotary multi-surface mirror) is used to effect the main scan with a constant period. However, this method is also disadvantageous in that since the optical deflector is additionally employed, the manufacturing cost is increased. Also, the arrangement of the entire system is rather intricate.